In the prior art, it has been recognised that an effective method of detecting an object intruding into or through a specified area is by monitoring an array of light beams spanning that area. Such an arrangement of optical paths as are known in the art can be created by the sequential activation of pairs of infra-red transmitters and receivers. Sequential high speed scanning of the infra-red transmitters and receivers effectively creates a screen of beams. In the event that an obstruction breaks one of more beams, an intrusion condition is sensed and appropriate action can be taken. In the case of elevator doors, such an obstruction event would normally activate a relay which halts or reverses the movement of the door.
Such elevator door sensing systems must be able to detect the presence of passengers or obstacles between the doors at all door positions and numerous systems have been proposed to achieve this.
Many systems known in the art use infra-red transmitters and sensors to establish the screen of beams in the area between the lift doors. Systems such as these can be prone to interference from ambient light impinging on the sensors. This results in spurious noise signals which can render the door obstruction detection system inoperative leading to inconvenience to lift passengers.
It is well known in the art to strobe the emitter/receiver array thereby creating a `screen` or `wall` of optical paths in the area between the doors. However, this requires communication across the door gap so that triggering or detector selection pulses may be communicated from a controller to the components of the emitter/detector array. This is conventionally achieved by means of a physical electrical connection between the doors, such as a cable. By necessity, this type of mechanical arrangement is usually movable as it is attached between the door units or a single door and slamming post. Any such connection is therefore prone to wear from repeated open/close cycles of the elevator doors.
Also, many prior art detection techniques operate by merely detecting either the absence or presence of an optical signal and triggering the door-opening mechanism accordingly. This does not allow for the possibility for small or variable signals being valid optical signals (ie.; those corresponding to an unbroken beam) and therefore such detection systems can produce unnecessary obstruction triggering events. Variable signals can be the result of aging of optical emitters and detectors, deterioration of the optical properties of the mounting construction and variations in the ambient optical conditions in the obstruction detection area.
In situations where the elevator doors are close, ie.; just prior to closure, the light beam intensity in the obstruction detection area may be sufficiently high that a hand or a similar obstruction will not break the light beam and no door obstruction event will be triggered. Human flesh does not attenuate high intensity infra-red wavelengths completely, therefore this effect is particularly significant if infra-red transmitters are used.
Another difficulty is that an obstruction which is close to a reflective surface in the detection area, can produce an optical signal resulting in spurious detection signals. Such a reflective surface might be the sill of the car door. It is therefore desirable to be able to distinguish between reflection, low intensity signals, and direct signals.